Absorption cooling makes sense

Each area and application is different, but in many places the absorption solution has become much more attractive and will bear reconsideration over an electric chiller.

Gas Technology

08/19/2015

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Sometimes it makes sense to review the current validity of choices made in the past, because basic premises may have changed. A good example of this is previous decisions to choose electric chillers over absorption machines. Each area and application is different, but in many places the absorption solution has become much more attractive and will bear reconsideration for your next chiller selection.

Using heat for cooling

Absorption chillers use the thermal energy in hot water or steam to power a reaction that chills water to feed a central chilled water cooling system. This technology is time-tested and used in facilities around the world. In recent years, some of the advantages of absorption have become more prominent, making it a very attractive choice for applications where, 20 years ago, electric chillers were the rule.

Broad USA is one of several providers of absorption chillers in the United States and Canada, and offers a wide range of machine types in sizes from 40 to 3,300 refrigeration tons. According to Doug Davis from Broad, there currently is increasing interest in absorption chillers for hospitals, schools and universities, and some large industrial energy users. One such industrial user is Roche Pharmaceuticals. Davis notes, "Roche has elected to no longer use chemical refrigerants on its sites, so they are selecting absorbers for their facilities in the U.S.A." Why are these kinds of major energy users attracted to the absorption process?

Perhaps the first consideration is that the operating costs of electric chillers have increased dramatically in many areas. Electric utilities not only have significantly increased energy rates, but also are increasing electric demand charges. Demand charges are calculated in various ways, but the most typical is the use of peak usage during a 15 minute interval each month. In many facilities, the demand charge may represent 30% to 70% of the monthly bill during the summer months.

Avoid peak demand charges

In many commercial, industrial and institutional facilities, electric chillers make the single greatest contribution to those summer peaks, when demand charges are usually the highest. New electric chillers are more efficient, and certain other strategies can help reduce that demand contribution, but electric chiller demand is still a major contributor.

Absorption chillers, on the other hand, have a comparatively small electric demand—just that needed for pumps and perhaps cooling tower fans. An absorption chiller typically has electric demand less than 10% of its same-sized electric alternative. With rising demand charges, it can be a money-saving choice.

Energy source may already be there

Another aspect worth considering is that absorption chillers can often use byproduct hot water or steam from other manufacturing or electric generation equipment on the site. The energy input is heat, and if you don't use it, you lose it. The absorption chiller should be selected and sized based on the flow and temperature of this byproduct heat. According to Davis, single-stage absorbers can effectively use hot water at temperatures as low as 190 F. He adds that single-stage steam absorption chillers can use steam at pressures as low as 5 psig, and two-stage machines as low as 60 psig.

Further, even if you can't use a byproduct heat stream, many facilities have steam or hot water boiler capacity already on site that is underused during the cooling season. It's frustrating to see an industrial plant venting steam because the boiler's minimum firing rate is greater than the plant demand for steam. By using this capacity to supply a chiller, you not only avoid the expense of a dedicated boiler, but you allow those existing boilers to operate more efficiently.

Moderate natural gas prices

We've talked about rising electric energy and demand charges, often with increases greater than the general rate of inflation. With natural gas as the thermal source, your facility can take advantage of the stability of natural gas prices. In some locations, you may even be able to take advantage of interruptible natural gas rates during the summer season, when interruptions are usually rare. Work with your natural gas utility to get the optimum rate.

Another place absorption is a great choice is when you have installed a natural gas-fired combined heat and power (CHP) electric generation plant on your site. Generator engines and especially gas turbines have significant potential for byproduct heat that can supply an absorption chiller with some or even all of its needed thermal input. The same CHP heat stream that provides building heat in the winter can provide building cooling in the summer.

New absorption chiller features

Yet another reason why absorption is become even more attractive is the designs of newer absorption chillers, with higher operating efficiency, greater reliability than even before, and simplified operation and maintenance requirements. Davis notes that another benefit is that with today's low-pressure absorption chillers, it is not necessary to have an operating engineer on duty while the chiller is operating. This is a huge benefit in locations such as New York City.

According to a report from the U.S. Department of Energy, absorption chiller systems running on waste heat can be a cost-effective replacement for another chiller type. The report recommends determining the cost-effectiveness by taking the following steps:

Conduct a plant survey to identify sources and availability of waste steam.

Determine cooling load requirements and the cost of meeting those requirements with existing mechanical chillers or new installations.

In situations where there is not a water or steam capability on site, manufacturers offer direct-fired absorption chillers, with steam generation capability included in the chiller package. The boiler section is closely optimized to the chiller section. In the winter, this same boiler can provide steam for space heat and humidification. Direct fired chillers are available in a variety of sizes from several manufacturers.

Remember to include the absorption option

Don't assume that an electric chiller is the only option. Is absorption the right choice? It is essential to do a complete evaluation, and get help from an engineer who has familiarity with absorption systems. Don't rely on decisions made 10 or 20 years ago. The rules have changed, and many of them now favor the absorption option.

Justification for considering absorption chillers over electric ones have been outlined in the article very well. Especially when any facility considers combined heat and power options, analysis should be made to see if absorption chiller operation is cost effective.

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